This invention relates to a following control apparatus for an automotive vehicle that can control the operation of a vehicle so as to maintain a safe separation from a preceding vehicle.
Recently, so-called following control apparatuses have been developed for automotive vehicles. A following control apparatus resembles a cruise control apparatus in that it controls the speed of a vehicle without the driver having to operate either the accelerator pedal or the brake pedal. However, whereas a typical cruise control apparatus merely maintains the vehicle speed constant, regardless of the movement of other vehicles on the same road, a following control apparatus controls the vehicle speed so as to maintain the separation of the vehicle from a preceding vehicle at an optimum level. The term "preceding vehicle" is used to refer to another vehicle travelling in front of and in the same direction as the vehicle being controlled.
A conventional following control apparatus includes a distance sensor for sensing the separation of the vehicle from a preceding vehicle. A target drive force for the vehicle is calculated on the basis of the measured vehicle separation, a target vehicle separation, and the relative speeds of the vehicle and the preceding vehicle. A target throttle opening corresponding to the calculated target drive force is calculated, and the throttle valve of the engine of the vehicle is controlled so as to attain the target throttle opening and therefore the calculated target drive force.
The target drive force is usually calculated using an equation such as the following one: EQU Target Drive Force=K.sub.1 .times.(measured vehicle separation-target vehicle separation)+K.sub.2 .times.relative speed (1)
wherein K.sub.1 and K.sub.2 are predetermined constants.
A conventional following control apparatus performs satisfactorily when the vehicle and the preceding vehicle are traveling at nearly constant speed, such as when the vehicle is traveling on a highway. The following control apparatus makes it unnecessary for the driver to pay careful attention to his separation from the preceding vehicle, so the strain of driving is greatly decreased.
However, a conventional following control apparatus has difficulty controlling a vehicle that is starting from a standstill. When a vehicle is stopped in traffic, the separation between the vehicle and the preceding vehicle is almost never equal to the target vehicle separation, and the separation varies randomly from situation to situation. If the target drive force at starting is calculated using the above Equation (1), the target drive force will depend on the vehicle separation at the time of starting. Even if the acceleration of the preceding vehicle as it moves from a standstill is the same in two different situations, the target drive force given by Equation (1) depends on the vehicle separation at the time of starting, so the acceleration of the vehicle being controlled can greatly vary in the two situations.
When the initial vehicle separation is smaller than the target vehicle separation, the effect on the calculated target drive force of the term in Equation (1) proportional to (measured vehicle separation-target vehicle separation) is greater than the effect of the term proportional to the relative speed, so even though the preceding vehicle is starting, the vehicle being controlled does not start. When the preceding vehicle further advances and the vehicle separation increases, the vehicle being controlled begins to move, but by that time, the speed of the preceding vehicle is already high, so in a short time, the vehicle separation becomes large. Due to the large vehicle separation, the vehicle being controlled undergoes sudden acceleration. As a result, the driver of the vehicle initially sees the preceding vehicle pulling away from his vehicle, so he has the subjective feeling that the following control apparatus has not performed properly. When his vehicle subsequently undergoes a sudden acceleration, the driver not only is feeling an extremely unpleasant physical sensation, but he may experience a fear that his vehicle is about to collide with the preceding vehicle. Furthermore, due to the sudden increase in vehicle separation at the time of starting of the preceding vehicle, the vehicle separation may become too large for the distance sensor of the following control apparatus to measure the vehicle separation, in which case following control becomes impossible.
It is possible to increase the responsiveness of the following control apparatus at the time of starting by increasing the gain of the apparatus, but this results in hunting (i.e., the periodic variation in speed of the motor with respect to current) of the vehicle when it reaches a normal driving speed, and the ride becomes uncomfortable for the passengers of the vehicle.
Thus, a conventional following control apparatus can not be utilized except when the vehicle being controlled and the preceding vehicle are moving at a nearly constant speed. When a vehicle with a conventional following control apparatus is moving in stop and go traffic, the following control apparatus can not be turned on until the vehicle reaches a fairly constant speed, so the driver must continually restart the apparatus after each time the vehicle stops in traffic. When frequent starts and stops are performed, the need for the driver to repeatedly restart the following control apparatus is troublesome, so the purpose of the apparatus, which is to decrease driver fatigue, is not achieved. Furthermore, if the driver inadvertently forgets to restart the following control apparatus but is under the impression that the apparatus is actually turned on, the driver is likely to pay inadequate attention to the separation from the preceding vehicle, and a collision with the preceding vehicle can easily result.